Fast active scanning wireless network apparatus and method

ABSTRACT

In a fast active scanning wireless network apparatus and method for quick determination of available access points ( 20 ), information about a candidate set of available access points ( 20 ) is obtained, and a candidate access point is identified from the candidate set. A mobile station ( 10 ) then queries the candidate access point with a probe request that designates the candidate access point as a sole responder. The probe request prevents other access points from contending for the medium of communication between the mobile station and the designated sole responder access point by excluding the attempt by other access points ( 20 ) to transmit probe responses. The apparatus and method thus increases the probability of a fast and successful probe request from the mobile station and subsequent response from the designated access point ( 20 ). The designated access point may also respond with a probe response of high priority, preventing intervention of communication.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application Nos.60/466,259, filed Apr. 29, 2003, and 60/470,228, filed May 13, 2003. Thepresent application incorporates the disclosure of these provisionalapplications by reference.

FIELD OF THE INVENTION

This invention relates to a method, apparatus and system for fast activescanning of a wireless network, for instance, fast active scanning ofthose networks certified under one or more of the IEEE 802.11 wirelesslocal area network (LAN) standards of interoperability, WPAN (WirelessPersonal Area Networks), networks certified under one or more of theIEEE 802.16 and 802.16a standards, Bluetooth™ networks (including butnot limited to IEEE 802.15.1 standard), HomeRF™ networks, HIPERLAN™networks, IrDA™ networks and/or other wireless networks.

BACKGROUND OF THE INVENTION

Wireless networks generally include multiple access points for wirelessconnectivity to multiple mobile stations. Such connectivity allows amobile station to communicate with any number of types of devices withina network, for instance, a mainframe, a server, a networked printer,another mobile station, and the like. Mobile stations determine whichnetwork to join by scanning for available channels. Scanning may beeither an active or passive on one or more radio channels.

Passive scanning is carried out by a mobile station simply listening forsignals broadcast from access points attached to the network. Activescanning is performed when a mobile station actively broadcasts a proberequest signal or the like. A probe request signal is meant to solicit aprobe response signal from available access points, based on which themobile station is then able to gain access to the network. Theinterchange of the probe request and probe response signals is generallyreferred to as a “handshake.”

When a mobile station is moving out of range of one access point andpotentially into range of another access point, a handoff between thoseaccess points occurs. Conventionally, such handoffs have been timeconsuming. Different problems can result from either of handshake orhandoff latency, ranging from interruption of communication all the wayto lack of communication between a mobile station and a network's accesspoints.

For instance, communications interruptions can result from TransferControl Protocol (“TCP”) congestion-avoidance algorithms kicking-inbecause of TCP-imposed time limits, causing decreased throughput betweenmobile stations and the network. Such latency can cause seriousdeterioration in voice transmission quality, for example, when handshakelatency causes voice service interruption time to exceed 35 msec. Asexplained in greater detail below, handshake latency can easily exceedpermissible limits, potentially causing a total loss of communication.

FIG. 1 is a diagrammatic example of a wireless local area network(“WLAN”), in which multiple mobile stations 10 wirelessly communicatewith access points 20 via wireless signals 30. Access points 20 act asmediators between the mobile stations 10 and devices already connectedto the network in either wired or wireless fashion (e.g., a server 40, acomputer 50, a networked printer 60, or other mobile stations).

In order to communicate with the network, the mobile station 10 firstsenses the wireless medium 30 to determine if another mobile station istransmitting over a particular channel. If the channel is determined notto be busy, the mobile station proceeds with an attempt to communicatewith the network on that channel.

The mobile station's attempt to communicate is orchestrated by, forexample, a Distributed Coordination Function (“DCF”) which may determineusing techniques, such as Carrier Sense Multiple Access with CollisionAvoidance (“CSMA/CA”), when any number of mobile stations are operatingwithin the network, as well as when those mobile stations should bepermitted to transmit and receive frames over the wireless medium.

Using one or more algorithms in accordance with the DCF, the CSMA/CArequires that a gap of a specified duration (otherwise called anInterframe Space, or “IFS”) exists between all contiguous sequences oftransmitted data. A transmitting device (whether an access point on thenetwork or a mobile station) is required to ensure that the wirelessmedium is idle for a specific duration (the duration being specified bythe type of IFS, as explained herein) prior to attempting to transmit.An IFS differs in length according to the priority of the frames beingtransmitted.

For example, a Short Interframe Space (“SIFS”), which is the shortestinterframe gap, is employed when a transmitting device has seized achannel on the wireless medium and needs to keep the channel for theduration of the frame exchange to be performed. One such SIFS example isthe acknowledgement frame (otherwise known as the “ACK signal” or “ACKframe”). Using the smallest gap between transmissions within the frameexchange sequence prevents other transmitting devices (whether accesspoints or mobile stations) from attempting to use the seized channelbecause the other transmitting devices have to wait for the channel tobe idle for a period of time that is longer than the SIFS period.Accordingly, the DCF and CSMA/CA provide priority to the currentframe/data exchange sequence over attempts by other transmitting devicesto transmit new data exchange sequences over the same channel on thewireless medium.

Another example of an interframe space is the Point Interframe Space (or“PIFS”). A PIFS is only used by transmitting devices operating under aPoint Coordination Function (“PCF”) to gain priority access to a channelon the wireless medium at the start of a contended free period. A PIFSgap is longer than a SIFS gap.

A Distributed Interframe Space (“DIFS”), which is longer than a PIFSgap, is used by transmitting devices operating under the DistributedCoordination Function (“DCF”) and CSMA/CA to transmit data exchangeframes and management frames. Management frames include such frames asprobe requests and probe responses, which are used for communicationshandshakes between a mobile station and an access point.

When transmitting under the DCF, if a mobile station determines that achannel on the wireless medium is busy, the mobile station delaystransmitting any new frames until the end of the current transmission.After this delay, or before attempting to transmit immediately followinga successful transmission, the station selects a random backoff intervaland decrements a backoff interval counter while the channel on thewireless medium is idle. The station reattempts transmission when thebackoff interval counter reaches zero.

Further general parameters of the various IEEE 802.11 WLAN standardsconcern frame receptions/acknowledgments, physical and virtual carriersense functions, and frame types. These parameters provide transportfunctions that ensure an intended receiver receives the total amount ofbits and bytes in a data message correctly. These further parameters aregenerally discussed below with respect to FIGS. 2-7.

In FIG. 2, the basic field of a probe request signal may include thefollowing: frame control, duration, destination address (“DA”), sourceaddress (“SA”), basic service set identification (“BSSID”), sequencecontrol, service set identification (“SSID”), supported rates, and framecheck sequence (“FCS”). Of these fields, the first six (frame control,duration, DA, SA, BSSID, and Sequence Control) are part of what is knownas the Media Access Control (or “MAC”) header for controllingtransmission over the wireless medium. The next two fields (SSID andsupported rates) constitute the probe request body. The final field(FCS) is used for error-detection.

The MAC header derives from the data link layer (otherwise known as theMedia Access Control layer, or “MAC” layer) of the network. This layerprovides a virtual carrier-sense mechanism (also known as a networkallocation vector, or “NAV”), for maintaining a prediction of futuretraffic on the medium based on duration information that is available inthe MAC headers of all transmitted information, with few exceptions.

The network may include a physical layer for providing a physicalcarrier-sense mechanism, which is based on energy detection in thewireless medium. In combination, the physical and virtual carrier-sensemechanisms help to determine the state of the wireless medium.

When attempting to communicate, mobile stations send probe requestsignals (including the MAC header) to scan the area for an existingnetwork channel, and to solicit a probe response from a network accesspoint (“AP”). Receiving APs use the probe request body (including theSSID and supported rates fields) to determine whether the mobile stationcan join the network. The mobile station desirably supports the datarates required by the AP, and indicates a desire to join any network orthe network identified by the SSID.

Further, as noted above, general receiving rules under the 802.11standards include a mobile station using a DA (destination address)field to perform address matching for receiving decisions. In the caseof the DA field containing a group address (e.g., a broadcast address)when the field is other than a beacon field, the BSSID must bevalidated. That is, the BSSID field must have the same BSSID as therecipient. The BSSID field can be the broadcast BSSID if the field is aprobe request. All transmitting devices (including APs and mobilestations) receiving data or management fields with a DA field other thana group address, respond to the received data or management fields withan acknowledgement signal (or “ACK” signal) transmitted with a SIFSdeferral. However, if received fields have a group address in the DAfield, ACK fields are not transmitted.

FIG. 3 is a diagram showing commonly incorporated bits that make up thecontrol field of a probe request. Type bits (B2, B3) and subtype bits(B4-B7) of the control field are used for identification of the fieldtype. Table 1 provides examples of valid type and subtype combinationsas used in the field control field of the probe request signal.

TABLE 1 Examples of Valid Type and Subtype Combinations Type SubtypeValue value B3 B2 Type description B7 B6 B5 B4 Subtype description 00Management 0100 Probe request 00 Management 0101 Probe response 00Management 1000 Beacon 00 Management 1101 Action 00 Management 1110-1111Reserved 01 Control 1101 Acknowledgement (ACK)

FIG. 4 shows the basic fields of a probe response signal, including aMAC header (for instance those MAC header fields described above inrelation to the probe request signal) and a frame body. The frame bodyincludes a timestamp, a beacon interval, capability information, theSSID, supported rates, data set parameters (“DS” parameters), controlframe parameters (“CF” parameters) and frame check sequence (FCS).

The probe response comprises fields for informing the mobile stationtransmitting the probe request of the network's characteristics,enabling the mobile station to match parameters so as to be able to jointhe network. Similarly to the fields of a probe request, type bits (B2,B3) and subtype bits (B4-B7) of the probe response's frame control fieldidentify the frame type. Table 1 provides examples of valid type andsubtype combinations as used in the frame control field of the proberesponse signal.

FIG. 5 shows the fields of the acknowledgement (or “ACK”) signal. ACKsignals are used to send positive acknowledgments in response toreceived frames. Similarly to the fields of a probe request and proberesponse, type bits (B2, B3) and subtype bits (B4-B7) of the ACK's framecontrol field identify the frame type. Table 1 provides examples ofvalid type and subtype combinations as used in the frame control fieldof the ACK signal.

FIG. 6 is a flowchart depicting the probe request portion of a handshakewhen scanning a network for available channels. The mobile stationbegins by discerning whether there are any unscanned channels in thenetwork. If all channels have been scanned under the DistributedCoordination Function, the process is complete.

When there are unscanned channels, the mobile station transmits a proberequest to a selected channel while simultaneously clearing and startinga probe timer clock (also known as “ProbeTimer”). The mobile stationthen senses the wireless medium to determine if the medium is busy untila minimum channel time is reached (the minimum channel time is alsoknown as “MinChannelTime”).

If the wireless medium is not busy during MinChannelTime, the channel ismarked as scanned and the mobile station returns to querying whetherthere are any unscanned channels in the network. If all channels havebeen scanned under the Distributed Coordination Function, the process iscomplete. If any unscanned channels remain, the mobile station transmitsanother probe request to a selected channel while simultaneouslyclearing and starting the ProbeTimer clock. The mobile station thensenses the wireless medium to determine if the medium is busy untilMinChannelTime is reached.

If the wireless medium is busy during MinChannelTime, then the stationwaits until the ProbeTimer clock signal reaches a maximum channel time(also known as the “MaxChannelTime” signal). Additionally, if anyunicast frame is received at the mobile station, the unicast frame isresponded to with an acknowledgement signal (also known as an “ACK”signal) from the mobile station.

When the MaxChannelTime is reached, any received probe responses areprocessed, and the probed channel is marked as scanned. The process ofFIG. 6 continues until all unscanned channels have been scanned.

FIG. 7 is a flowchart showing the procedure used by access points (APs)for sending out a probe response in answer to a received probe requestsignal. Proper probe request signals include a broadcast destinationdesignated by the destination address (“DA”) field, a broadcast BSSID,and probe request fields that satisfy the particular reception rules forthe particular access points (“APs”) receiving the probe request. Onreceiving probe request fields, the APs respond with a probe responseonly if the SSID in the probe request is the broadcast SSID, or if theSSID in the probe request matches the specific SSID of a particular AP.Probe response frames (for example, those response frames with thecharacteristics as described above in relation to FIG. 4) are sent asdirected frames to the address of the mobile station that generated theprobe request. On receipt, the mobile station acknowledges them with anACK signal.

If a probe response is not acknowledged with an ACK signal, the proberesponse is retransmitted for a predetermined number of attempts (thelimit on number of retransmission attempts is also known as theRetransmissionLimit). The probe response follows the normal rules of theDCF (as previously described). APs transmitting a probe response arecontinuously maintained in an awake state, and respond to all proberequests meeting the above-noted criteria.

In FIG. 7, a counter (for counting the number of attempts before an ACKsignal is received in response to a transmitted probe response) iscleared at the beginning of the process. Then, received probe requestsare checked to see whether the request's SSID is the broadcast SSID, orwhether the SSID of the probe request matches the AP's SSID. If neitherof these criteria is met, a probe response is not sent, and the processends. If either of these criteria is met, the counter is checked to seewhether the retransmission limit is less than the predeterminedretransmission limit. If the retransmission limit has been reached, anadditional probe response is not sent, and the process ends.

If the retransmission limit has not been reached, then a probe responseis sent under the normal rules of DCF. The AP then waits for receptionof an acknowledgement (“ACK”) signal. If the ACK signal is receivedbefore an acknowledgement timeout (“AckTimeOut”), then the process iscomplete. If an ACK signal is not received prior to the AckTimeOuttiming out, the counter is incremented and the process begins again withthe comparison of the retransmission limit to the counter value, asdescribed above, until the process has completed with either theRetransmissionLimit being reached, or reception of an ACK signal beforethe AckTimeOut timing out.

FIG. 8 shows an example of an overall handshake between a mobile stationand an AP, from initial probe request to subsequent probe response andacknowledgement signal transmission. In FIG. 8, a first mobile station(“STA 1”) is scanning for availability of a specific channel on anetwork, for example, channel 1. STA 1 contends for the medium ofchannel 1 under the basic access rules of DCF by transmitting a proberequest signal with a broadcast destination address and broadcast BSSID.Since the probe request has been broadcast, any APs in the physicaltransmission area of the probe request that are using channel 1 willreceive the probe request signal and will attempt to respond with theirown probe response signals. For instance, AP 1 and AP 2 shown in FIG. 8will each attempt to respond with a probe response.

When AP 1 and AP 2 transmit their own probe responses, AP 1 and AP 2contend for the medium under the basic access rules of the DCF, asexplained above. Further, AP 1 and AP 2 are also contending with othertransmitting devices (e.g., STA 2 as shown in FIG. 8) attempting to sendtheir own frames under the DCF. Frames thus colliding are known topotentially cancel one another out, or to cancel one or the other out.

A mobile station that has sent a probe request frame determines theavailability of the APs based on the medium status as reflected by thereceipt of probe responses within the minimum channel time (also knownas the MinChannelTime). A probe timer (also known as “ProbTimer”) tracksthe time for response to the probe request. If the wireless medium isidle until the ProbTimer reaches MinChannelTime, the mobile stationconsiders that no AP is available, as reflected by cases 5 and 7 inTable 2.

TABLE 2 Determination of AP Availability Through Probing Transmission ofReceipt Medium busy Probe Response Receipt Determined Availability ofProbe during during of Probe Availability Judgment of AP RequestMinChannelTime MaxChannelTime Response of AP Correctness Case 1 ◯ ◯ ◯ ◯◯ ◯ ◯ Case 2 ◯ ◯ ◯ ◯ X X X Case 3 ◯ ◯ ◯ X X X X Case 4 ◯ X ◯ X X X XCase 5 ◯ ◯/X X ◯/X X X X Case 6 X X ◯ X X X ◯ Case 7 X X X X X X ◯

Conversely, if the wireless medium is not idle as determined by receiptof a probe response within the MinChannelTime period, the AP having sentthe probe response is considered as available to serve the mobilestation (for instance, case 1 of Table 2). Otherwise (as shown by cases2, 3, 4, and 6 of Table 2), the mobile station considers that no APs areavailable. However, the judgment that no AP is available is not alwayscorrect, as explained below.

In Table 2, correct judgment is reflected by cases 1, 6 and 7. In case1, the AP is available and successful receipt of a probe response signalverifies this. In case 6, no AP is available. That is, either the AP ormobile station are out of the physical transmission area and either theprobe request or probe response cannot be delivered to the recipient.This fact is derived from a failure to receive a probe response withinthe time limit of MaxChannelTime.

In case 7, no AP is available. That is, either the AP or mobile stationare out of the physical transmission area and there fails to be anotheractive station transmitting in the vicinity. MinChannelTime thus expireswithout the mobile station detecting the wireless medium as busy.

Further in Table 2, incorrect judgment is reflected in cases 2, 3, 4 and5. In case 2, the probe request was received correctly, and the proberesponse was sent before expiration of MaxChannelTime. However, themobile station could not receive the probe request correctly. Reasonswhy this may occur include the probe response signal colliding withframe transmissions of other stations or other APs. In case 3, the proberequest has also been received correctly, but the probe response signalcannot be sent prior to MaxChannelTime expiring. This may be due to theAP not being able to get a timely transmission opportunity for a proberesponse, or because of medium contention with other stations or APs, oras a result of other data frames that precede the probe response in thetransmission queue. In case 4, the AP could not receive the proberequest correctly. The probe request could have collided with the frametransmissions of other mobile stations or APs. In case 5, MinChannelTimehas been set to an improper value that is smaller than is actuallyrequired.

In cases 2 through 5 of Table 2, the decision that no APs are availableis incorrect. A scanning mobile station in conventional scanning mode isthus provided three options for handling these problems: (1) use of alarger MinChannelTime, (2) use of a larger MaxChannelTime, or (3)retransmission of the probe request signal.

In any of the above-three options, an increase in scanning time istraded for the associated increase in accuracy. The problem of handshakelatency resulting from any of the above-three options is only multipliedby the plurality of channels to be scanned, as there typically is morethan one channel to be scanned in a wireless network. Conventionalmethods are accordingly extremely inefficient in terms of scanning time,the result being high handshake latency. As explained above, problemsresulting from high handshake latency vary from mere interruption ofcommunication all the way to utter non-communication between a mobilestation and a network's access points.

SUMMARY OF THE INVENTION

Therefore, one feature of the present invention provides a method,network and system for fast active scanning of a wireless network,including collecting candidate access point information thatdifferentiates between candidate access points which are most likely toprovide communication with wireless stations in the network andcandidate access points which are not; and transmitting the candidateaccess point information. It is also a feature of the present inventionto select, from the candidate access point information, the candidateaccess points that have been differentiated as most likely to providecommunication; and transmitting the differentiated candidate accesspoint information.

It is a further feature of the present invention to scan, with awireless station, the candidate access points that have beendifferentiated as most likely to provide communication. It is yet afurther feature of the present invention that the scanning of candidateaccess points includes the wireless station transmitting a probe requestfor a specified candidate access point chosen from among the candidateaccess points differentiated as most likely to provide communicationwith the network.

Additional features of the present invention are evident from theassociated drawings and in the Detailed Description of Embodimentssection, below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, both as to its organization and manner of operation, maybe further understood by reference to the drawings that include FIGS.1-27, taken in connection with the following detailed description.

FIG. 1 is an illustration of a wireless local area network;

FIG. 2 is an example of the basic fields included in a probe requestsignal;

FIG. 3 is a diagram showing commonly incorporated bits that make up thecontrol field of a probe request;

FIG. 4 is an example of the basic fields included a probe responsesignal;

FIG. 5 is an example of the basic fields included in an acknowledgementsignal;

FIG. 6 is a flowchart which shows the basic procedure of sending proberequest signals;

FIG. 7 is a flowchart which shows the basic procedure of sending proberesponse signals;

FIG. 8 is a temporal illustration showing a handshake between a mobilestation and an access point on a wireless network, including sending aprobe request, a probe response and acknowledgement signals;

FIG. 9 depicts fields of candidate access point (“CAP”) information;

FIG. 10 depicts bit-frame information for the scanning capability of atypical AP;

FIG. 11 depicts an AP with a bit assignment for B8 that indicates thatthe AP is provided with the fast active scanning of the instantinvention;

FIG. 12 depicts a neighborhood graph showing the relationship of each APto the other;

FIG. 13 is a flowchart that depicts a first embodiment of the instantinvention;

FIG. 14 depicts an exchange of frames in a handshake according to thefirst embodiment;

FIG. 15 is a flowchart that depicts a second embodiment of the instantinvention;

FIG. 16 depicts an exchange of frames in a handshake according to thesecond embodiment;

FIG. 17 is a flowchart showing the procedure that a CAP follows in thesecond embodiment after reception of a probe request;

FIG. 18 is a flowchart illustrating a third embodiment of the instantinvention;

FIG. 19 is another flowchart illustrating the third embodiment;

FIG. 20 depicts an exchange of frames in a handshake according to thethird embodiment;

FIG. 21 is a flowchart which depicts fast active scanning according to afourth embodiment of the instant invention;

FIG. 22 is a flowchart that illustrates the procedure of a CAP (for thefourth embodiment) after receiving a probe request;

FIG. 23 depicts an exchange of frames in a handshake according to thefourth embodiment;

FIG. 24 is a flowchart illustrating a fifth embodiment of the instantinvention;

FIG. 25 depicts an exchange of frames in a handshake according to thefifth embodiment (incorporating elements of the third and fourthembodiments);

FIG. 26 depicts an exchange of frames in a handshake according to thefifth embodiment (incorporating elements of the second embodiment);

FIG. 27 is an example of frames including the format of action frames;and

FIG. 28 illustrates an example of a processor or processors that may beused in the candidate access points and the mobile stations of theinstant invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The following description of illustrative non-limiting embodiments ofthe invention discloses specific configurations and components. However,the embodiments are merely examples of the present invention, and thus,the specific features described below are merely used to describe suchembodiments and to provide an overall understanding of the presentinvention.

Accordingly, skilled artisans readily recognize that the presentinvention is not limited to the specific embodiments described below.Furthermore, the descriptions of various configurations and componentsof the present invention that are known to one skilled in the art areomitted for the sake of clarity and brevity.

Further, while the following embodiments employ the IEEE 802.11standards by way of example, the present invention may be applied todifferent wireless networks, including but not limited to thoseenumerated above.

The present invention reduces scanning time and handshake latency bymaking handshakes less susceptible to misjudgment without increasing thenumber of times a probe request is retransmitted and also withoutincreasing either of MinChannelTime or MaxChannelTime. Indeed thepresent invention is capable of reducing handshake time when there is nodesignated AP by allowing the use of a small MinChannelTime.

In brief, the instant invention incorporates information on neighboringAPs to focus a probe request to a specific AP, chosen from a list ofcandidate access points (“CAPs”). CAPs are determined by providing themobile station with prior knowledge of the likely state of existing APs.FIG. 9 is an example of the information provided to mobile stationsconcerning candidate access points.

CAP information includes respective addresses and operational channelnumbers for each CAP provided in the list. Further, if CAPs of multipleextended service sets (ESSs) are listed, then the SSID of the respectiveCAP may be included to facilitate appropriate CAP selection by themobile station. Additionally, if a plurality of methods are deployed foractive scanning, information on the CAPs' scanning capability may alsobe included to facilitate the selection of a scanning method by themobile station to match the CAPs' capabilities.

FIG. 10 depicts bit-frame information of the scanning capability of atypical AP. The depicted bit-frame information shows assignment of thecapability information field. These frames may be transmitted in beaconor probe response signals, or in other signals.

FIG. 11 depicts an AP with a bit assignment for B8 that indicates the APis provided with the fast active scanning of the instant invention. Anyreserved bit space could be used to denote fast active scanning. Thedepicted use of B8 in FIG. 11 is purely an example. Furthermore, morethan one reserved bit could be used to indicate the specific scanningcapability.

Information on CAPs can be collected and delivered to the mobilestation(s) in any of various fashions. One such method is offlinecollection by an operator, in which network providers or operators cancollect information by examining coverage of each AP offline. Thismethod of collection of information may be extracted/implemented duringthe original planning for all APs.

Another method is real time collection by all APs, using reassociationmessaging, in which mobile stations that make a handshake between twoAPs are recommended to send a reassociation frame containing informationof prior APs to subsequent APs. Subsequent APs are then required to senda move-notify message containing information about themselves to priorAPs. Through such message exchanges, APs can themselves collectinformation on CAPs.

Another real time collection method for information on CAPs would bescan reporting, in which mobile stations that have finished scanning canbe requested by the current AP to send a scanning results report of theprevious scan to the current AP.

Because information collected by the two above-noted methods(reassociation messaging and scan reporting) may be constrained withinthe boundaries of direct-neighbor APs, a corresponding real time methodfor collecting CAP information is information exchange among existingAPs. This method could be incorporated with either or both ofreassociation messaging and scan reporting to exchange information amongall APs in a network. FIG. 12 depicts how exchanging information amongall APs results in a neighborhood graph of the APs that shows theirrelationship to each other.

A further real time collection method could be the collection of CAPinformation by a mobile station. That is, results of prior scans can beutilized for subsequent scans. For instance, when beginning an initialscan, mobile stations could use the above-noted information on CAPs asprovided by an offline operation or as provided by real time collectionby APs. Another option would be to simply use conventional scanning asan initial scanning method, and then to switch to a scanning method ofthe instant invention as provided herein.

If CAP information is collected by entities other than a mobile station,the information needs to be delivered to the station. Information may bedelivered in any of several ways, as follows, by way of example.

Delivery of CAP information may be performed via beacon or proberesponse for an existing AP to a receiving mobile station. Sending theCAP information in a beacon signal may result in a relatively long lagtime as a beacon signal is typically sent periodically. Additionally, amobile station receiving a probe response is generally more interestedin the availability of the AP itself (rather than neighboring APs), socontaining the CAP information in a probe response may result inunnecessary overhead.

Delivery of CAP information may also be done by request and response.That is, only when a mobile station requests the CAP information will itbe delivered. The CAP information may be sent to the specific requestingmobile station, or to the entire network.

Delivery of CAP information may also be performed by simply uploadingthe information to each station during initial setup.

Collection and delivery of CAP information may be performed in any ofthe above-described methods, or in combinations of the above-describedmethods, or via other methods. Further, the following embodiments may becombined in various ways.

In the following embodiments, the instant invention employs the CAPinformation that has been collected and delivered (as described above)to the mobile station attempting to communicate with the network.Accordingly, mobile stations attempting to communicate with the networkare therefore enabled to tune to the operational channel of a specificCAP in practicing the various embodiments.

Embodiments of the invention are enumerated below. The numbering doesnot imply any order of importance. All of these embodiments depictvarious aspects of the invention.

I. First Embodiment

In a first embodiment, the mobile station uses the CAP information thathas been collected and delivered to the mobile station to differentiatebetween those access points that are most likely to providecommunication (CAPs) and those access points which are not likely toprovide communication (e.g., busy or otherwise non-communicative APs).This allows the MaxChannelTime to be set to less than in conventionalscanning methods.

In comparison to conventional scanning techniques (e.g., the scanningtechnique disclosed in FIG. 6), the process of the first embodimentinvolves the directed scanning of specific APs (candidate access points)vis-à-vis the scanning of network channels (as disclosed by FIG. 6, forinstance).

The first embodiment accomplishes directed scanning of available CAPsthrough utilization of the provided CAP information, using the receivedCAP information to enter specific access point identificationinformation in the fields of probe requests for individual CAPs. Thismay be done by entering the address of a specific CAP in the BSSID fieldalong with the SSID of the CAP in the SSID field of the probe requestsignal. Alternatively, this may also be done by inserting the address ofthe CAP in the BSSID field along with the broadcast SSID in the SSIDfield of the probe request signal. Table 3, below, reflectspossibilities in specifying permissible field values for the proberequest signal of the first embodiment.

TABLE 3 Permissible Field Values for the Probe Request in the FirstEmbodiment DA BSSID SSID Type 1 Broadcast address Address of the AP SSIDof the AP Type 2 Broadcast address Address of the AP Broadcast SSID

In this embodiment, the CAP that has received a probe request signalwill respond with a probe response only if the SSID in the probe requestis the broadcast SSID or if the SSID field matches the specific SSID ofthe CAP. Probe response frames will be sent as directed frames to theaddress of the mobile station that generated the probe request and willbe subsequently acknowledged by the mobile station. This exchange offrames is illustrated in FIG. 14. Notably, only the CAP that has theaddress of the BSSID field will be able to respond to the probe requestframe. Other APs, although operating on the same channel or having thesame SSID, will not respond to the probe request because the proberequest does not designate them as a responder.

FIG. 13 is a flowchart that depicts the first embodiment. In FIG. 13,the mobile station begins by receiving CAPs information (wherein theCAPs information differentiates between those access points most likelyto enable communication and those access points that are not) and thenspecifying a list of CAPS that are to be examined based on those mostlikely to enable communication.

Next, the mobile station discerns, from the list of CAPs to be examined,if any unscanned CAPs are present. If all CAPs have been scanned underthe Distributed Coordination Function, the process is complete.

When there are unscanned CAPs, the mobile station transmits a proberequest to a selected CAP while simultaneously clearing and starting aprobe timer clock (also known as “ProbeTimer”). The mobile station thensenses the wireless medium to determine if the medium is busy until aminimum channel time is reached (the minimum channel time is also knownas “MinChannelTime”).

If the wireless medium is not busy during MinChannelTime, the CAP ismarked as scanned and the mobile station returns to querying whetherthere are any unscanned CAPs in the network. If all CAPs have beenscanned under the Distributed Coordination Function, the process is thencomplete.

If any unscanned CAPs remain, the mobile station transmits another proberequest to a selected CAP while simultaneously clearing and starting theProbeTimer clock. The mobile station then senses the wireless medium todetermine if the medium is busy until MinChannelTime is reached. If thewireless medium is busy during MinChannelTime, then the station waitsuntil the ProbeTimer reaches a maximum channel time (also known as the“MaxChannelTime” signal). Additionally, if any unicast frame isreceived, it is responded to with an acknowledgement signal (also knownas an “ACK” signal) from the mobile station.

When the MaxChannelTime is reached, any received probe responses ateprocessed, and the probed CAP is marked as scanned. The process of FIG.13 continues until all unscanned CAPs have been scanned.

FIG. 14 depicts a temporal exchange of frames in the first embodimentbetween a mobile station and a CAP. A scanning mobile station sends aprobe request signal to a CAP. After the DIFS period and a randombackoff interval, the CAP responds with a probe response signal. After asubsequent SIFS period, the mobile station confirms receipt of the proberesponse with an acknowledgement (“ACK”) signal.

II. Second Embodiment

In a second embodiment, MinChannelTime and MaxChannelTime can both beset to less than required for conventional scanning systems, for thereasons explained herein. The mobile station of the second embodimentcan learn of the actual availability of a CAP not only through thereceipt of a probe response signal from the CAP, but also, if the mobilestation is only interested in knowing if the CAP is available, throughreceipt of an ACK signal. Accordingly, a probe response is notnecessary.

That is, instead of (or in addition to) a probe response, the mobilestation may be alerted to the availability of a CAP through receipt ofan ACK signal sent from the CAP. Conversely, if the mobile station isinterested in more information than just the availability of the CAP, itmay wait for a probe response signal to be transmitted subsequent to thetransmission of an ACK signal from the CAP. Otherwise, the mobilestation may proceed to the scanning of other CAPs without waiting for aprobe response.

The procedure of the second embodiment of the invention is shown by theflowchart of FIG. 15. As FIG. 15 is similar in nature to FIG. 13,similar features will not be reiterated.

FIG. 15 shows that for an ACK signal received in response to atransmitted probe request, the CAP in question is marked as scanned andavailable, and then either the remaining CAPs to be scanned are scannedor the process is at an end because all CAPs have been scanned.

Conversely, if an ACK signal is not received in response to a proberequest, the CAP is simply marked as scanned and then either theremaining CAPs to be scanned are scanned or the process is at an endbecause all CAPs have been scanned.

The second embodiment accomplishes directed scanning of available CAPsusing the provided CAP information to enter specific access pointidentification information in the frames of probe requests forindividual CAPs. This may be done by entering the address of a specificCAP in the DA field in the probe request signal. Either of the addressof a specific CAP or the broadcast BSSID is entered in the BSSID field,along with either of the SSID of the CAP or the broadcast SSID in theSSID field of the probe request signal. Table 4, below, reflectspossibilities in specifying permissible field values for the proberequest signal of the second embodiment.

TABLE 4 Permissible Field Values for the Probe Request in Embodiment 2DA BSSID SSID Type 1 Address of the AP Address of the AP SSID of the APType 2 Address of the AP Broadcast BSSID SSID of the AP Type 3 Addressof the AP Address of the AP Broadcast BSSID Type 4 Address of the APBroadcast BSSID Broadcast BSSID

Frame exchange in the second embodiment is altered so that transmittedprobe request signals are responded to with an ACK signal. Whiledifferent from conventional scanning methods, the second embodimentstill complies with the general rules for IEEE 802.11 acknowledgements,since the probe request maintains non-group destination addressassignment, as shown in Table 4, above. Notably, in the secondembodiment (and additionally in the first embodiment) only the CAP thathas the address of the DA field will be able to respond to the proberequest frame. Other APs, although operating on the same channel orhaving the same SSID, will not respond to the probe request because theprobe request does not designate them as a responder.

An entire frame exchange of the second embodiment is illustrated by FIG.16. As shown in the figure, a probe request is sent to a specific CAP,and the designated CAP responds with an ACK signal. It should beunderstood that the subsequent probe response and ACK depicted in dottedline as shown in the figure is purely an optional feature of the secondembodiment.

FIG. 17 is a flowchart showing the procedure that a CAP follows in thesecond embodiment after reception of a frame request. In the figure,CAPs that receive probe requests that are in compliance with Table 4respond with an ACK signal. As an optional feature, if the SSID in theprobe request is the broadcast SSID or if the SSID matches the specificSSID of the CAP, then the CAP initiates transmission of a probe responseas directed frames to the address of the mobile station that generatedthe probe request. The probe response is sent using the normal frametransmission rules of the DCF.

In the second embodiment, MinChannelTime can be set to a value not muchgreater than the SIFS value, since an ACK signal is sent in response tothe probe request immediately after the SIFS interval.

III. Third Embodiment

FIG. 18 is a flowchart illustrating a third embodiment of the instantinvention. In the figure, a mobile station in fast active scanning modeis first provided with CAP information, as previously described. Thestation then specifies a list of CAPs that are to be examined based onthe provided CAP information. Next, the station waits either until aProbeDelay time has expired or until an indication that an incomingframe has been received.

The mobile station then sends a probe request signal similar to thatshown in FIG. 2, but with the DA, BSSID and SSID fields filled withinformation as provided in Table 3. That is, the broadcast address willbe entered in the DA field, the address of the CAP will be entered inthe BSSID field, and either of the SSID of the CAP or the broadcast SSIDwill be entered in the SSID field of the probe request. The ProbeTimeris then cleared and begun. If the medium has not been detected as busybefore the ProbeTimer reaches MinChannelTime, then the next CAP isscanned. Otherwise, any received probe responses are acknowledged andprocessed. Then, the next CAP is scanned. When all CAPs in the CAP listhave been examined, the scanning process ends.

Notably, MinChannelTime in the third embodiment may be reduced to thatnot much greater than a SIFS value or even to the same value as theACKTimeout.

FIG. 19 is a flowchart depicting the third embodiment of the instantinvention. In FIG. 19, the CAP receiving the probe request signalresponds with a probe response only if the SSID in the probe request isthe broadcast SSID or if the SSID in the probe request matches thespecific SSID of the CAP. What differentiates the third embodiment fromother embodiments (such as the first and second embodiments), however,is that the transmission of the probe response is commenced immediatelyafter an IFS period that is smaller than the DIFS period.

Although either of a SIFS or PIFS period may be employed in the thirdembodiment, the following description incorporates the use a SIFSperiod. (If a PIFS period is used, MinChannelTime should be set to avalue not much greater than the PIFS period.) Accordingly, thedesignated CAP in the third embodiment has higher priority access to thewireless medium due to a shortened IFS period.

The probe response frame is acknowledged with an ACK signal sent by thescanning station. An entire frame exchange of the third embodiment isdepicted in FIG. 20. In the Figure, a probe request is sent to aspecific CAP. The minimum channel time has been reduced to the SIFSperiod, after which the CAP sends a probe response. After a further SIPSperiod, the probe response is acknowledged by the mobile station with anACK signal.

Determination of availability of the CAP in the third embodiment isperformed as follows. If the wireless medium is idle until ProbeTimerreaches MinChannelTime, the mobile station thus discerns that thedesignated CAP is not available in the area. If the wireless mediumbecomes busy before ProbeTimer reaches MinChannelTime, and the receivedprobe response is from the designated CAP, the CAP is regarded asavailable to serve the mobile station. If the wireless medium becomesbusy before ProbeTimer reaches MinChannelTime, and the received signalis other than the probe response signal from the designated CAP, the CAPis regarded as unavailable.

IV. Fourth Embodiment

While the third embodiment obeys the general receiving rule of the IEEE802.11 WLAN standards that directed frames should be acknowledged withan ACK signal and that frames with group destination addresses are notacknowledged, the fourth embodiment loosens some of the generalreceiving rules of the IEEE 802.11 standards, allowing for moretime-efficient scanning.

FIG. 21 is a flowchart that depicts the fast active scanning of thefourth embodiment. In the figure, the mobile station is provided withCAP information from which a specific list of CAPs to be examined iscreated. If there are any CAPs in the list that have not been scanned, aCAP is selected for scanning and the mobile station transmits a proberequest to the selected CAP (simultaneous to clearing and starting aProbeTimer). In the transmitted probe request, the DA, BSSID and SSIDfields are filled in with information corresponding to that shown inTables 3 or 4.

If the wireless medium is not detected as busy prior to the ProbeTimerreaching MinChannelTime, then the next CAP is scanned. Otherwise, anyreceived probe responses are processed. Subsequent CAPs are scanneduntil all CAPs in the list have been scanned.

In the fourth embodiment, MinChannelTime is a value only large enough toensure the successful detection of frames transmitted using SIFS or PIFSperiods, and preferably is set to a period not much greater than a SIFSperiod (if the probe response is sent with a SIFS period), or set to notmuch greater than a PIPS period (if the probe response is sent with aPIFS period). In a non-limiting version of the fourth embodiment, theprobe response signal is sent with a SIFS period, and MinChannelTime isset to the same value as ACKTimeout.

In compliance with the general receiving rules of the IEEE 802.11 WLANstandards, only the access point designated by the scanning stationreceives the probe request signal that is sent with the DA, BSSID andSSID fields filled in according to the information provided in Tables 3and 4. With further reference to FIG. 21, the CAP receiving the proberequest signal responds with a probe response only if the SSID in theprobe request is the broadcast SSID or if the SSID in the probe requestmatches the specific SSID of the CAP.

Probe response signals are sent, immediately after an IFS period that issmaller than a DIFS period, as directed frames to the address of thestation that generated the probe request. Although either a SIFS or PIPScould be used, the fourth embodiment is described herein as using a SIFSperiod.

The probe response signal sent by the recipient of the probe requestsignal may be regarded as successful acknowledgement of the proberequest signal. The probe response signal, however, is neitheracknowledged nor expected to be acknowledged. The mobile station thusproceeds with scanning the next CAP in the list of CAPs withouttransmitting an ACK signal to the CAP that has transmitted a proberesponse.

FIG. 22 illustrates the procedure of a CAP (for the fourth embodiment)after receiving a probe request. In the figure, if a probe request'sSSID is the broadcast SSID or if the probe request's SSID matches theCAP's SSID, then a probe response is sent after either a SIFS or PIFSperiod.

FIG. 23 depicts an entire frame exchange of the fourth embodiment. Inthe figure, the CAP responds to a response request with a probe responsesignal after a SIFS (or PIFS) period. Determination of availability ofthe CAP in the fourth embodiment is performed as follows. If thewireless medium is idle until ProbeTimer reaches MinChannelTime, themobile station thus discerns that the designated CAP is not available inthe area. If the wireless medium becomes busy before ProbeTimer reachesMinChannelTime, and the received probe response signal is from thedesignated CAP, the CAP is regarded as available to serve the mobilestation. If the wireless medium becomes busy before ProbeTimer reachesMinChannelTime, and the received signal is other than the ACK signalfrom the designated CAP, the CAP is regarded as unavailable.

V. Fifth Embodiment

When transmitting a probe request in conventional methods, a scanningstation must contend for the wireless medium under the access procedureprovided by the DCF. Although this ensures fairness of opportunity amongany transmitting stations, it is also time-consuming. Therefore, in thefifth embodiment of the instant invention, after transmitting a firstprobe request signal under the basic rules of the DCF, a scanningstation is then allowed a higher priority transmission opportunity (thenumber of which may be restricted in fairness to other mobile stationswhich may be attempting to transmit their own probe request signals).

The fifth embodiment is a modification of embodiments 2, 3 and 4 and isshown by the flowchart depicted in FIG. 24. In the fifth embodiment, aSIFS period is used for the IFS period between a probe request signaland probe response signal. If the SIFS period lapses without the receiptof the expected probe response signal, the mobile station is thenallowed to resend the probe request signal as soon as one PIFS periodafter transmission of the probe request. This permits the scanningstation to retain control of the wireless medium until an allowedretransmission limit (RetryLimit) has been reached.

An entire frame exchange of the fifth embodiment with a RetryLimit of 1is illustrated in FIG. 25 (incorporating elements of previouslydescribed embodiments 3 and 4) and FIG. 26 (incorporating elements ofpreviously described embodiment 2). In FIG. 25 and FIG. 26, an initialprobe request signal has not been responded to within the PIFS period byan appropriate probe response and ACK, respectively. Accordingly, themobile station retransmits another probe request at the end of the PIFSperiod. As further shown in the figures, an appropriate probe responseand ACK is received after the second transmission of the probe request.An acknowledgment signal may or may not be transmitted in response tothe probe response signal in FIG. 25.

VI. Sixth Embodiment

In a sixth embodiment of the invention, employment of the fast activescanning as provided in any of the previous embodiments is made optionalin conjunction with conventional scanning. By way of explanation,conventional scanning and fast active scanning (per disclosedembodiments of the invention) use different values in the DA and BSSIDfields of a probe request. That is, conventional scanning uses broadcastdestination and broadcast BSSID in the probe request signal, while theinvention uses the address of the CAP in the DA or BSSID fields.

Therefore, the sixth embodiment deploys conventional scanning techniquesif either the broadcast destination or the broadcast BSSID is used inthe DA and BSSID fields of a probe request, respectively. Conversely, ifthe address of the CAP is used in either the DA or BSSID fields of theprobe request, the CAP uses fast active scanning (as provided herein bydeployment of an embodiment of the invention). In deploying either ofthe scanning methods, the probe request uses types bits of 00 andsubtype bits of 0100 in the frame control field.

VII. Seventh Embodiment

In the seventh embodiment of the invention, scanning methods aredistinguished by explicit indication in the header of the probe requestsignal. In this embodiment, the explicit notification desirably usesreserved type bits and subtype bits of the frame control field asdepicted in Table 1. More desirably, the explicit notification usesreserved subtype bits for the management field.

For example, type bits of 00 and subtype bits of 1110 in the framecontrol field are assigned to indicate the active scanning according tothe second embodiment of the invention, while type bits of 00 andsubtype bits of 1111 are assigned to indicate the fast active scanningof the fourth embodiment of the invention. As in Table 1, type bits of00 and subtype bits of 0100 in the frame control field are used toindicate conventional active scanning. Therefore, in the seventhembodiment, the active scanning method to be deployed by the CAP isdetermined as follows.

If the probe request, in the frame control field, has the pair of typebits and subtype bits that are assigned to indicate a specific fastactive scanning method, the CAP uses the indicated fast active scanningmethod. The CAP will allow probe request signals of assigned type andsubtype bits even before authentication or association, withoutfiltering.

VIII. Eighth Embodiment

In an eighth embodiment of the invention, scanning methods aredistinguished by explicit indication in the header and frame body ofprobe request signals. Preferably, the explicit notification usespredetermined fields in the frame body of the action frame (i.e., typebits 00 and subtype bits 1101 in the frame control field).

The action frame provides a mechanism for specifying extended managementactions. The format of the action field is shown by example in FIG. 27.The category field will be set to one of the non-reserved values shownbelow in Table 5.

TABLE 5 Category Values Name Value Reserved 0-2  Radio measurement 3Reserved 4-127 Error 128-255 

The action details field contains the details of the action. The detailsof the actions allowed in each category include a radio measurementcategory with action field values, for instance, those values depictedin Table 6, below.

TABLE 6 Radio measurement action field values Action field valueDescription 0 Measurement Request 1 Measurement Report 2-255 Reserved

In any reserved category, values may be used to explicitly indicate thespecific fast active scanning to be used. For example, a category valueof 4 may be assigned to indicate the fast active scanning according tothe second embodiment, while a category value of 5 may be assigned toindicate the active scanning of the fourth embodiment. Therefore,determination of the fast active scanning method to be deployed isdetermined as follows. In the predetermined header or frame body of aprobe request, if the specific value that is assigned to indicate aspecific fast active scanning method is found, the CAP uses theindicated fast active scanning method. The CAP will not filter out proberequest signals of assigned type and subtype bits even beforeauthentication or association.

Additional Embodiments

FIG. 28, which is similar in nature to FIG. 1, depicts an embodiment ofthe invention including at least one processor 21 that, as part of acandidate access point 20, is capable of processing candidate accesspoint information that differentiates between candidate access pointswhich are most likely to provide communication and candidate accesspoints which are not, according to one or more of the embodiments of theinvention described herein.

FIG. 28 also depicts a further embodiment of the invention including atleast one processor 11 that, as part of a wireless station 10, iscapable of processing candidate access point information thatdifferentiates between candidate access points which are most likely toprovide communication and candidate access points which are not, alsoaccording to one or more of the embodiments of the invention describedherein.

Although probe responses are described herein as directed frames sent tothe scanning station, the probe responses could also be a broadcastframe or a multicast frame to a set of stations. Additionally, thefields in the probe response described herein for informing the scanningstation of the characteristics of the AP are not always required intheir listed entirety. Some or all of the fields can be removed.Furthermore, although specific frame formats are depicted herein forboth the probe request and probe response, the invention is notconstrained by any specific frame format. Further, the terms “field” and“frame” may be used analogously herein.

Also, the IEEE standards described herein use a network allocationvector (“NAV”) for directed frame transmission to secure the channel forthe subsequent atomic frame transmissions. This can be used for any ofthe embodiments described herein without change. In embodiments 3, 4 and5, NAV can even be used for the probe request signal with a broadcastdestination address, since these embodiments expect effectively only onerecipient (even though they are broadcast). The NAV value for the proberequest in embodiments 3, 4 and 5 could thus be set to the sum of SIPSand the transmission time of the probe request and probe response with aminimum size.

Moreover, as previously noted, the invention is herein explained throughuse of the IEEE 802.11 WLAN standards. The invention may readily beapplied to other wireless communication systems, including but notlimited to those mentioned throughout the specification.

Advantageous characteristics in the embodiments of the present inventioninclude reduced scanning time and handshake latency while simultaneouslyreducing handshake susceptibility to misjudgment without increasing thenumber of times a probe request is retransmitted and also withoutincreasing either of MinChannelTime or MaxChannelTime.

The previous description of embodiments is provided to enable a personskilled in the art to make and use the present invention. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art, and the generic principles and specific examplesdefined herein may be applied to other embodiments without the use ofinventive faculty. For example, some or all of the features of thedifferent embodiments discussed above may be deleted from theembodiment. Therefore, the present invention is not intended to belimited to the embodiments described herein but is to be accorded thewidest scope defined only by the claims below and equivalents thereof.

What is claimed is:
 1. A method of enabling active channel scanning in awireless network, said method comprising: collecting candidate accesspoint information that differentiates between candidate access pointsthat are available for association; selecting a candidate access pointfrom said candidate access points based on said candidate access pointinformation; and transmitting a probe request, said probe requestincluding an address of said selected access point that is specified ina basic service set identification field, wherein, if said selectedcandidate access point does not respond to said probe request within aninterframe space period that immediately follows said probe request andthat interframe space period is shorter than a distributed interframespace period, said wireless station resends said probe request in thetime period after said interframe space period that immediately followssaid probe request has ended but prior to the time period that adistributed interframe space period would have ended if said distributedinterframe space period had begun at the same time as said interframespace period that immediately follows said probe request.
 2. A wirelessstation in a wireless communications network comprising: a collectorwhich collects candidate access point information that differentiatesbetween candidate access points that are available for association; aselector that, based on the candidate access point information, selectsa candidate access point from said candidate access points; and atransmitter for transmitting a probe request, said probe requestincluding an address of said selected candidate access point that isspecified in a basic service set identification field, wherein, if saidselected candidate access point does not respond to said probe requestwithin an interframe space period that immediately follows said proberequest and that interframe space period is shorter than a distributedinterframe space period, said wireless station resends said proberequest in the time period after said interframe space period thatimmediately follows said probe request has ended but prior to the timeperiod that a distributed interframe space period would have ended ifsaid distributed interframe space period had begun at the same time assaid interframe space period that immediately follows said proberequest.
 3. A wireless access point among a plurality of access pointsin a telecommunications system that are available for association,comprising: a receiver that receives a probe request from a wirelessstation, said probe request including an address of said wireless accesspoint that is specified in a basic service set identification field; anda transmitter for transmitting a response to said probe request,wherein, if said wireless access point does not respond to said proberequest within an interframe space period that immediately follows saidprobe request and that is shorter than a distributed interframe spaceperiod, said wireless station resends said probe request in the timeperiod after said interframe space period that immediately follows saidprobe request has ended but prior to the time period that a distributedinterframe space period would have ended if, said distributed interframespace period had begun at the same time as said interframe space periodthat immediately follows said probe request.
 4. A wirelesstelecommunications system comprising a plurality of access points,wherein at least one of the access points comprises a processor whichprocesses candidate access point information that differentiates betweencandidate access points that are available for association; and awireless terminal which, based on said candidate access pointinformation, selects one of said access points and transmits a proberequest, said probe request including an address of said candidateaccess point that is specified in a basic service set identificationfield, wherein, if said selected access point does not respond to saidprobe request within an interframe space period that immediately followssaid probe request and that interframe space period is shorter than adistributed interframe space period, said wireless station resends saidprobe request in the time period after said interframe space period thatimmediately follows said probe request has ended but prior to the timeperiod that a distributed interframe space period would have begun ifsaid distributed interframe space period had begun at the same time assaid interframe space period that immediately follows said proberequest.
 5. A wireless telecommunications system comprising: a pluralityof wireless stations, wherein at least one of the wireless stationscomprises a processor which processes candidate access point informationthat differentiates between candidate access points that are availablefor association; and a wireless station that, based upon said candidateaccess point information, selects an access point and transmits a proberequest, said probe request including an address of said selected accesspoint that is specified in a basic service set identification field,wherein, if said selected access point does not respond to said proberequest within an interframe space period that immediately follows saidprobe request and that is shorter than a distributed interframe spaceperiod, said wireless station resends said probe request in the timeperiod after said interframe space period that immediately follows saidprobe request has ended but prior to the time period that a distributedinterframe space period would have ended if said distributed interframespace period had begun at the same time as said interframe space periodthat immediately follows said probe request.